fix: normalize OIDC issuer URL to avoid double slashes

The OIDC issuer URL in .env ends with a trailing slash, which caused
the authorize endpoint to have a double slash (//authorize).

- Normalize issuer URL by removing trailing slash before appending path
- Applied to both LoginPage.tsx and CallbackPage.tsx
- Fixes Authentik 'not found' error on login redirect

apps/web/src/pages/LoginPage.tsx

@@ -20,7 +20,8 @@ export default function LoginPage() {
         code_challenge_method: 'S256',
       })
 
-      window.location.href = `${OIDC_ISSUER}/authorize?${params.toString()}`
+      const issuer = OIDC_ISSUER.endsWith('/') ? OIDC_ISSUER.slice(0, -1) : OIDC_ISSUER
+      window.location.href = `${issuer}/authorize?${params.toString()}`
     }
 
     initiateLogin()